U.S. military advisers in Syria had to call in American airstrikes and artillery support to repel an attack late Wednesday night by pro-Assad regime forces on a headquarters belonging to U.S.-backed rebels fighting ISIS. An estimated 100 pro-regime forces were killed by the strikes, said a U.S. official.

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A U.S. official said there were no U.S. casualties in the attack, which the U.S.-led anti-ISIS coalition called "unprovoked" and an "act of aggression" by "Syrian pro-regime forces."

A coalition statement said the attack occurred on a "well-known" headquarters belonging to the U.S.-backed Syrian Democratic Forces (SDF) about eight kilometers east of the deconfliction line along the Euphrates River that is used to separate the U.S. area of operations from that of Russia and the Assad regime.

American troops look out toward the border with Turkey from a small outpost near the town of Manbij, northern Syria Feb. 7, 2018.

"Syrian pro-regime forces initiated an unprovoked attack against well-established Syrian Democratic Forces headquarters Feb. 7," said the statement. "Coalition service members in an advise, assist, and accompany capacity were co-located with SDF partners during the attack."

"In defense of Coalition and partner forces, the Coalition conducted strikes against attacking forces to repel the act of aggression against partners engaged in the Global Coalition's defeat-Daesh mission," said the statement. Daesh is an Arabic acronym used to describe ISIS.

The estimated 100 pro-regime fighters were killed when American airstrikes and artillery support were called in to repel the attack, said a U.S. official.

The attack on the SDF headquarters east of the Euphrates River was well coordinated and involved Russian-made T-54 and T-72 tanks, as well as artillery.

The Associated Press

US Army Capt. Timothy Skinner, left and Lt. Gen. Paul E. Funk consult at an American outpost in the northern Kurdish town of Manbij, Syria, Wednesday, Feb. 7, 2018.

The U.S. official said that pro-regime forces who turned back across the Euphrates River were not engaged by U.S. aircraft.

The U.S.-led coalition used a telephone hotline to contact the Russian military and advise them of the defensive airstrikes in the area.

"The Coalition remains committed to focusing on the defeat-Daesh mission in the Middle Euphrates River Valley and asserts its non-negotiable right to act in self-defense," the statement continued.

A U.S. official said the American advisers helped defend the attack on the SDF headquarters located south of Raqqa in the Middle Euphrates River Valley. That valley region east of Raqqa is the last holdout for the several thousand ISIS fighters still on the battlefield.

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The American advisers called in air strikes and artillery support to help push back the attackers, said the official. There were no U.S. casualties in the incident, according to the official.

The term "pro-regime forces" has been used in the past by the coalition to describe Syrian militia forces loyal to the Assad regime, as well as Iranian-led groups of Hezbollah fighters supporting the regime.

Wednesday's attack represents one of the most brazen violations yet by pro-regime forces of the Euphrates River deconfliction line.